1. Field of the Invention
The invention relates to detection of neutrons and more specifically, it relates to a semiconductor neutron detector that utilizes a lithium compound as the sensing element.
2. Description of Related Art
Neutrons are uncharged elemental particles that do not provide direct energy transfer by ionizing matter as they pass through it. The velocity of neutrons increases with their energy. High energy neutrons interact with the light nuclei (H, He, Li) by transferring a fraction of their energy in multiple collisions (the so called recoil reaction) until they reach an energy close to thermal (0.026 eV). This process is called thermalization. The signal developed by this reaction is very weak and difficult to distinguish from the signal developed by gamma interaction, therefore this type of interaction usually is not used for neutron detection but only to slow down the neutrons to the thermal energy where the probability is very high for another process called capturing. The capturing process takes place in special stable isotopes of some materials such as He-3, Boron-10, Lithium-6 and Uranium-235, where the thermal neutron splits the absorber atom in two particles that will have kinetic energies in the MeV range and velocities in opposite directions. The percentage fraction of captured neutrons versus total accidental neutrons defines the capturing detection efficiency
When the capturing of thermal neutron occurs, the charged particles produced by the capturing reaction transfer their energy by ionizing the capturing material and the charge collection material (gas or solid state semiconducting material) thus creating free charges. An applied electrical field separates the charges (electrons, holes and ions) and moves them toward the electrodes. The charge collected on the electrodes produces an electrical pulse signal with a given amplitude distribution (pulse height spectrum). An amplitude discriminator with an event detection threshold is used to separate the pulses of neutron origin from those originating from gamma or noise. The fraction of the pulses with amplitude above the detection threshold versus total neutron pulse height distribution defines the charge collection efficiency. The product of capturing and charge detection efficiency determines the overall intrinsic efficiency of the thermal neutron detector.
Some of the most popular thermal neutron detectors are gaseous detectors based on boron trifluoride (BF3) or helium three (3He). Here the capturing material is constituent in the gas molecule, therefore the same gas serves as a capturing medium as well as an ionization medium and a charge transport medium. The reaction particles deposit the whole energy in the gas volume thus providing near 100% charge collection efficiency. The combination of high capturing efficiency (typically about 70%) and high charge collection efficiency make these detectors a backbone of neutron detection technology.
Though boron trifluoride (BF3) or helium-3 (3He) gaseous detectors are very efficient, their application is limited by factors such as: a) high cost (3He is very expensive and is constantly depleted due to losses in the open space); b) difficulties for transportation because they are pressurized devices or use chemically aggressive BF3 gas; and c) gaseous detectors are bulky and cannot address handheld and other special applications (some examples described below).
Solid state capturing materials such as B-10, Li-6, U-235 and Gd can be used as a thin film adjacent to the ionization and charge collection/charge transport medium. The thickness of the capturing layer, thus the capturing efficiency, is limited by self-absorption inside the capturing layer. The thin layer is applied as a film or a dopant.
Only a charge particle traveling toward the ionization medium can produce a signal and the signal amplitude depends of the energy absorption in the capturing layer. Thus both the neutron capturing and charges collection efficiency are limited substantially compared to a 3He detector. Only a fraction of the charged particles deposit energy. The ionization medium can be a noble gas or solid state semiconductor material such as silicon, silicon carbide or germanium.
U.S. Pat. No. 3,227,876, incorporated herein by reference, is directed to a neutron detecting solid state device or the like. The patent discusses a silicon semiconductor having a layer doped with boron. Neutrons are absorbed by the boron layer, creating energetic reaction particles that, in turn, create electron-hole pairs that diffuse into and across the junction to produce an electric current pulse. The detector may be encapsulated by a few centimeters thick layer of hydrogenous moderator material in order to reduce the speed of incoming fast neutrons to create thermal neutrons for detection.
U.S. Pat. No. 6,388,260, incorporated herein by reference, is directed to a solid state neutron detector and method for use. The neutron detecting material is a lithium tetraborate or alpha-barium borate crystal. Neutrons are absorbed by the boron layer to create energetic reaction particles that create electron-hole pairs. The electron-hole pairs diffuse into and across the junction to produce a current pulse.
A problem with prior art neutron detectors is the sensitivity of the detector to gamma rays. Lithium glass scintillators, although generally less efficient, are an effective means for detecting low-energy neutrons and find wide application in neutron scattering research. However, lithium glass scintillators also suffer from sensitivity to gamma rays where the presence of a background radiation is large in relationship to a flux of neutrons. In such instances, the gamma sensitivity of lithium glass simulates a neutron capture event in lithium glass and there is no effective technique for separating the gamma signal from the neutron signal.